This invention relates to demodulation of angle (i.e., frequency of phase) modulated electrical carrier signals.
In so-called frequency shifted keyed ("FSK") modems, the transmitted information is represented by a succession of pulses of different frequencies. Various schemes have been used to detect the frequency of each incoming pulse. One such scheme, "differential detection", involves a correlation technique in which a discriminator forms the product of the incoming signal with a delayed signal to provide an output representative of the input frequency; e.g., in a digital implementation, successive samples of the incoming waveform are multiplied by earlier such samples stored in a delay line, to provide a duty cycle modulated output whose deviation from a 50/50 duty cycle is proportional to the deviation of the input frequency from a reference frequency. Typically, after removal of carrier frequency (and multiples thereof) components, this output is compared with a threshold to make a decision about the input frequency.
Williams U.S. Pat. No. 3,778,727 describes a differential detector, suitable for integrated circuit implementation, in which the received signal is delayed in a multistage shift register before being correlated, in an EXCLUSIVE OR gate, with a later received signal. The product of this correlation passes first to an integrator circuit and subsequently to a differential amplifier to produce a replica of the original binary modulating signal.
In Hellwarth et al., U.S. Pat. No. 3,571,712 a zero crossing detector converts an incoming FSK modulated carrier signal into a bilevel square pulse train which is delayed in a shift register, before being exclusively or'ed with an undelayed counterpart. The resulting signal passes to an up-down counter which counts to above a threshold value when the signal is mostly high and to below the threshold value when the signal is mostly low. The subject matter of Hellwarth et al., U.S. Pat. No. 3,571,712 is also described in Jones, G. D. Jr., "Digital Frequency Discriminator", IBM Technical Disclosure Bulletin, Vol. 13, No. 11, April 1971, pp. 3421-3422.